st: ask for help about ttesti

[rghuang <0034058@fudan.edu.cn>]

if i want to use

ttesti #obs1 #mean1 #sd1 #obs2 #mean2 #sd2 or ttesti #obs #mean #sd

but i just don't know what sd is,just know variance.how should i do?

do i have to use display to compute sd first?or still have some other ways?

thank you.